1. Field of Invention:
The present invention relates to catalysts, articles, and methods for treating combustion exhaust gas.
2. Description of Related Art:
Combustion of hydrocarbon fuel produces exhaust gas that contains, in large part, relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2). But the exhaust gas also contains, in relatively small part, noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of undesirable components, preferably by a process that, in turn, does not generate other noxious or toxic substances.
NOx, which includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), is a burdensome component to remove from exhaust gas generated by lean burn engines (e.g., diesel engines and gas turbines). The reduction of NOx to N2 is particularly problematic in lean burn exhaust gas because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. Notwithstanding, NOx can be reduced by a process commonly known as Selective Catalytic Reduction (SCR).
An SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a nitrogenous reducing agent, such as ammonia, into elemental nitrogen (N2) and water. In an SCR process, ammonia is added to an exhaust gas stream prior to contacting the exhaust gas with the SCR catalyst. The reductant is absorbed onto the catalyst and the NOx reduction reaction takes place as the gases pass through or over the catalyzed substrate. Chemical pathways for stoichiometric SCR reactions using ammonia include:2NO+4NH3+2O2→3N2+6H2O2NO2+4NH3+O2→3N2+6H2O
N2O is produced in limited quantities by combustion. However, undesirable processes occurring in SCR systems also include several competitive, nonselective reactions with oxygen, which is abundant in the system. These reactions can either produce secondary emissions including the production of N2O. Commercial SCR catalysts, including vanadia, iron-zeolites, and copper-zeolites, all produce N2O. Exhaust gas treatment systems, particularly in which the NO2 content has been increased to exceed the NO level in the feed gas, allow N2O formation via pathways such as:8 NO2+6 NH3→7 N2O+9 H2O4 NO2+4 NH3+O2→4 N2O+6 H2O2NH3+2O2→N2O+3H2OAccordingly, there remains a need for improved SCR catalysts that provide improved NOx conversion and reduced N2O make. The present invention satisfies these needs amongst others.